Sound recording



' June 6, 1939. J. H. MOLEOD ET AL SOUND RECORDING Filed Oct. 13, 1937 2 Sheets-Sheet l TRACK 251.

TRACK 25?? J m y R B Junes, 1939. J. H. MoLEOD ET AL 2,161,369

SOUND RECORDING Filed Oct. 13, 1937 2 Sheets-Sheet 2 Jo/m Mceod em m mm OE 0 MW/ m 5 a am Patente d June 6, 1939 PATENT OFFICE SOUND RECORDING John H. McLeod and Richard S. Morse, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application October 13, 1937, Serial No. 168,'777

10 Claims.

This invention relates to sound recording and particularly to push-pull sound-on-film recording wherein the soundrecord is divided or doubly recorded on two tracks which are usually immediately adjacent to each other on the film and in which the variations in light transmission are 180 degrees out of phase; e Although it is conflned to push-pull recording', the inventtion may be applied either to variable density or variable width systems. ?It is a primary object of the invention to provide a sound recording system which will permit variable density to be used with class B push-pull recording.

The term Class B Push-Pull" is used by those skilled in this art to refer to systems in which each half of the positive double sound track comprises a variable width which follows one half of each cycle of the signal being recorded, but which remains small and constant during the other half of each cycle. An example of such a track appears as Fig. 2 of the accompanying drawings. It will be noted that the two halves of the track are 180 degrees out of phase and hence when the width of one track is varying,

that of the other is small and constant. As is well known, Class B recordings have usually been produced by using a mask having two triangular openings-one inverted, the other upright with their vertices on the same horizontal line. This gives a ,double trlangular slot which when refiected from a galvanometer through a narrow ,slit forms'a double light beam of this Class B push-pull type. such systems need not be discussed in detail here especially since galvanometers do not lend themselves to variable density recordings as readily as do ribbon valve systems and fiashing lamp systems. Class A systems are those in which each of the two sound tracks 40 represent a full wave including both halves of each cycle.

It is an object of the invention to provide Class Bpush-pull sound tracks having variable density rather than variable width and further to provide apparatus using only electrical means for producing this Class B eifect.

It is a particular object of the invention to' 'produce such a sound record using flashing lamps.

The advantages of Class B recor dings are well 5 known. For example, such a recording is automatically "noiseless" in the sense in which this word is commonly-employed in this art, since the light transmission of the positive sound track tends toward zero for low signal intensities (low sound Volume). In Class A systems or non pushpull systems, so-called "noiseless recording' may be obtained by biasing the valve or a cooperating mask according to the total sound Volume. In such systems there is an inherent time delay so that the biasing action will not aifect the individual cycles. This permits a swishing noise when changing from a loud to a soft note or to a region of silence, during the brief interval before the noiseless feature takes eifect. This swishing effect is particularly noticeable with piano recordings wherein each note is momentarily loud. Class B recordings overcome all these dilliculties since the noiseless feature is automatic and the sound track width or density is determined solely by the particular cycle being recorded. Thus another of the advantages of the present invention is the combination of the well known advantages of variable density recordings with the advantages of Class B push-pun recording.

According to the invention, a half-wave rectifier is placed in series with each of the two light modulating devices which may be of the ribbon type, flashing lamp type, etc., or even of the galvanometer type although in the latter case there 'is little advantage gained over an optical mask such as those now in use.

It is an object of one special embodiment of the invention to combine push-pull amplification with push-pull recording and to use the electron discharge devices of the amplifier as the rectifiers to give "Class B Recordings". Thus all the advantages of push-pull amplification and of our broad invention are combined in a simple, convenient and newmanner.

It is an object of another special embodiment of the invention to provide a push-pull recording system using glow lamps which act as their own half-wave rectifiers. This may be accomplished by using two three-electrode glow lamps With their recording electrodes biased just below the extinction point and the third electrode biased to maintain a pilot glow. Thus the use of separate rectifying units is eliminated.

In any type of variable density sound recording, it is usually desirable to operate entirely on the straight line portion of the characteristic curve of the photographic emulsion, or at least to avoid the extreme lower end of the toe of this curve. One method of obviating any difiiculties in this connection is to expose uniformly (flash) the sound tracks beyond a desired minimum value so that all light variations due to sound modulations act in addition to this flash exposure and thus on a useful portion of the characteristic curve. As an added feature of the present invention when using either two-electrode or three electrode glow lamps, this flash exposure can be provided by the pilot glow. This is accomplished by adjusting the biasing potential which maintains the pilot glow to a value which produces suflicient intensity in this gloW to furnish the desired uniform flash exposure. p

Other objects and advantages will be apparent from the following description of detailed embodiments of the invention when read in connection with the accompanying -drawings in which:

Fig. 1 shows the invention applied to` ribbon type light valves.

Fig. 2 shows a section of a positive'sound track resulting from a recording of the type illustrated in Fig. 1.

Fig. 3 shows a system similar to that used in Fig. 1 for recording variable density sound tracks.

Fig. SA shows graphically the density variations of'the sound track shown in Fig. 3.

F'ig, 4 .and Fig. :'5 illustrates directly how 'the invention may 'be applied to two-electrode glow :lamps.

Fig. 6 illustrateshow the invention may be V combined -with push-pull amplification.

Fig. 7 illustrates the nvent'ion using threeelectrode glow lamps. r

7-Ai1lustrates in a simple manner *the principle of thearran-gement shownin Fig. '7.

In' 'FigQl -an input signal from 'a microphone M and its accompanying amplifying circuit H 'is introduce'd througha transformer having a -prim'ary I 2 and'a secondary !3. Two ribbon type light wa'lves 16 'and *1 1 are connected across this secondary +3, or may beconnected across a-'portion of it -or even have separate secondaries as shown *in Fig. 3. A 'half-wave rectifier 'l4 is in cl-uded *in series with the valve 16 and a halfwave rectifier I 5 is correspondingly included 'in the circuit of the valve 11. These rectifiers |4 'and 5 'are connected 'to have opposite directions 'of 'actionas'illustrated so'that the valve IE operates only 'onthepositive half of each cycle and the'valve .l' operates only on the negativehalf, or 'vice versa.

.An optical system including a lamp !8, lenses TS I'S'and `2`I, and ,a daphragm 23, projects an image o'f the aperture between 'the ribbons of eachlight valve to a sensitive film *22 on which the ;pictures may "be .recorded in areas '23 and .the sound .track is recordedin twoportio'ns 24 L and R. Itwill be noted that variationsin width in eitherhalf of the trackare ?contemporary with aregion of uniform width in the other half "In Fig f2, .the .positive film "22 carrying pictures !Tenda positive sound track.24' corresponding :with thesound *track 24 of Fig, :1, demonstrates quite clearly that .the light transmission .of the .positive sound track "tends toward .Zero for low intensity signals. This produces the desired noiseless efiect. i i

In Fig. 3 the lightvalves 6 and :H -areshown connectedtogether for convenience in manufact-ure a-nd 'hence must receive .their signal from separatesecondaries 26 and -2-1. *In this :particular arrangement-the optical system'includes separate masks 28 and 29 anda bi-prism':30 which is used to bring'the'two tracks to adjacentpositions ou -the film. -Any 'suitable optical system such a's t'hat shown may be -used to -produce a variable density push-*pun sound track 25 'on'the film 22. The density "variations in this track are shown in'Fig. `-3A. It will be noted that in 'this case also the variations in light transmission in one track are accompanied by constancy of light transmission in the other track. As in the previous 'case this constant density is low in the negative and hence high in the positive giving noiseless recording.

In Fig. 4 the invention is applied in a simple manner to glow lamps ,32 and 33, each of which is, biased just above the extinction point by a source of potential 3! through suitable resistances. For lamp 32 these resistances are 36 and 38; for lamp 33, theresistances are 31 and 39. The input signal from the'secondary 13 through the rectifiers ,M `and 14', and the resistance 34 tends on ,cne 'half of each cycle to increase the current flowing through the lamp 32 since the direction of rectification is the same as the direction of 'biasing for this lamp. Thus on one half of the cyclethe lamp 32 receives current through the rectifiers !4 and M' as well as from the source oi potential '31, whereas the lamp 33 is unable 'to receive any signal current due to .the rectifiers 15 and l5'. In this connection it should 'benoted that the source of 'potential almust be of a low resistance type so'thatthe output flowing through the rectifiersiland 14' is unable to cause any change of potential across th'e glow:1amp.33 *If the rectifiers :"4' 'and"|5'areremoved :from the circuit and the source-of. 'potential 3| is connected to a center topon the secondary |`-3 as shown by the broken line 13', a similar 'result would 'be obtained, -but the response of the glOW lamps Would -not ex'actlyfollow the 'half wave rectifica tion of the input signal. input signal; the lamp 3 2 would receive current "from the sourceof potential 31 through theresistance 36 and also through 'the upper half 26 of thesecondary During the positive half of an input signalthis biasing current would be in- 'crease d'as discussed above;'-but on thenegative half of *the -inputjthis bias'ing' current would not remain exactlyconstant but the portion 'flowing through {26 ;would be ;reduced-probably to ;zero 'However the' case' illustrated :using four rectifiers avoids this 'difiiculty completely. p

A 'somewhat 'more' fiexible arrangement for applyingthe'invention to glow 'lamps is shownin Fig. ''wherein'resistances 34 and 3'5 are arranged in a bridge circuit with resistances '40 and '41, These resistances are adjusted to give the desired biasing potential from' the source 3! across the lampsfiz *and `33^to maintain them just above the extinction point. Furtlermore, 'the ratio 'of the resistances'3'4 to '40, '35130 41 and-43 to ;42 are suchthat thepoints 44,45 and 46 are at the same potential for zero signal input. The resistances 43 and `42 may be omitted if the point 46 is connected to a tap on thesource ofpotential 3`l; .the tap being adjusted to 'hold this point "46' at the potential of 44 and'45. The operation of this circuit is similar to thatdescribed in connection withnFig. 4. However the extra ;rectifiers l4' and 15' are unnecessary since there is zero current through the secondaries 26 and 31 for Zero'input signal, whereas the omission of 14'` and 15' in Fig. 4introduced a disturbing factor as discussed. According to a Very preferable embodiment of the invention as shown in Fig. 6 the rectifiers [4 and 15 may be replaced by electron discharge devicesshown .as triodes 48 and,`4'9. circuitsof .these triodes are biased by a source of potential '41 so that there is zero output for zero input only. In the particular arrangementshown the glow`lamp biasing potential 3| also acts as the plate supply 'for each of the triodes. 'To per- ,The input In this. case for zero mit convenient adjustment of the resistances in the circuit of the triodes 46 and 49 the lamps 32 and 33 are shunted by variable resistances 50 and 5! 'respectively.

The arrangement shown in Fig. '7 using threeelectrode glow lamps and eliminating the use of any other type of half-wave rectifier is most easily explained by reference to Fig. 7A. A threeelectrode glow lamp 52 is arranged with 'its recording electrodes 54 and 56 across a secondary 26 of the input circuit. A resistance 34 is in series with these recording electrodes for added stability. A source of potential 66 and a resistance 64 are used to bias these recording electrodes just below the extinction point. 'I'he third electrode 58 of the glow lamp 52 is biased by a source of potential 68 and through a resistance 60 to maintain in the lamp a glow between this pilot electrode and one of the recording electrodes,- in the caseshown, electrode 56.

In the half of each cycle of the input signal which through the secondary 26 causes a potential across the electrodes 54 and 56 in the same direction as that caused by the biasing potential 66, a discharge between the two recording electrodes will occur sincethe presence of a pilot glow insures that the ignition and extinction points for the recording electrodes are the same.

In the other half of the cycle the output of the secondary 26 is in the opposite direction to the biasing potential 66 and hence tends to reduce the potential across the recording electrodes 54 and 56. Since the potential 66 is just at the extinction point, the glow between these two electrodes'goes out.

The other three electrode lamp 53 is biased to operate on the opposite half of each cycle. Since the purpose of the pilot electrode 58 is merely to maintain a pilot glow, it is immaterial whether the potential of this electrode is positive or negative with respect to the electrode 56, provided that the difference in potential .is sufficient to maintain the glow. In the three-electrode glow "lamps commercially available, the electrodes 54 and 58 appear to be similar and the electrode 56 is of the crater type.

In the simplified arrangement shown in Fig. '7 the four sources of `potential 66, 61, 68, and 69 are replaced by one source of potential 62. The Variable resistance 34 in addition to adding stability also provides the adjustment obtained by resistance 64 in Fig. 7A. The arrangement shown in Fig. 'ZA is not intended to be a practical one but is shown merely as an aid in discussing the very useful embodiment shown in Fig. 7.

As pointed out previously the pilot glow in either the two-electrode or three-electrode flashing lamps may be adjusted to have sufiicient intensity to give a uniform flash exposure of any desired intensity to the film.

The advantages of the arrangements shown in Figs. 6 and 7 over any of the previous ones described are quite apparent and illustrate why these arrangements are the preferable ones. A direct combination of the specific embodiments of the two figures is also quite useful but is not described here since in general it is unnecessary to use any additional rectifiers with the threeelectrode glow lamps.

Having thus described our invention and two particularly preferable embodiments of it, we wish to point out that it is not limited to the specific arrangements shown but is of the scope of the appended claims.

What we claim and wish to protect by Letters Patent of the United States is:

1. The method of Operating light valves for Class B push-pull sound recording which comprises receiving an electric signal corresponding to the sound to be recorded, half-wave rectifying in the positive direction a portion of the signal, modulating a light valve in accordance with this half-wave rectification, half-wave rectifying in the negative direction another portion of the signal and modulating another light valve in accordance with this latter half-wave rectification.

2. An electrc circuit for push-pull sound recording comprising an input circuit for introducing the signal to be recorded, two light modulating devices, a circuit for each device including the device and means connecting it across a portion of the input circuit, an electrical halfwave rectifying means in each device circuit for controlling the light modulating devices according to a half-wave rectification of the signal, the two rectifying means having opposite directions of action.

3. An electric circuit for noiseless sound recording comprising an input circuit carrying a full wave signal to be recorded, two light modulating devices adapted for push-pull recording, means connecting said input circuit to each of the light modulating devices and a half-wave rectifier in series with each of the two connecting means for controlling the light modulating devices according to a half-wave rectification of the signal, the two rectifiers being oppositely coupled in their respective circuits.

4. An electro-optical system for push-pun variable density sound-on-film recording comprising an input circuit for introducng the signal to be recorded, two light modulating devices, two light beams the intensities of which are modulated thereby, optical means projecting said modulated beams to different tracks on the film, and electrical means including half-wave rectifiers connecting each of the light modulating devices to the input circuit for controlling these devices according to a half-wave rectification of the signal, the two rectifiers being opposed and acting alternately.

5. An electric circuit for push-pull sound recording including an input circuit carrying the signal to be recorded, two glow lamps connected to the input circuit, said glow lamps having a plurality of electrodes, two electrodes in each lamp being biased just above the extinction point, the biasing of the two lamps being in opposite directions and two electrodes in each lamp forming a complete series circuit with a portion of the input circuit, said series circuit including means passing uni-directional current only, said last mentioned means for one lamp being in the opposite direction from that for the other lamp.

6. An electric circuit for push-pull sound recording including a signal input circuit, two flashing lamps, means including half-wave rectifying means connecting each lamp across a portion of the input circuit, and a source of potential biasing said lamps just above the extinction point and supplying current which maintains a glow in each lamp, said biasing and rectiiying means being for each lamp in the same direction and for either lamp in a direction opposite to that for the other lamp.

'7. An electric circuit for push-pull sound recording including an input circuit, two electron discharge devices connected in push-pull across the input circuit, a glow lamp across the output of each of the electron discharge devices. and means biasing each lamp just, above the extinction point in the same direction, as the output of the corresponding electron discharge device which is biased giving zero output for zero input only.

8. In push-pun sound recording, an input circuit carrying, a full wave corres nding to the sound to be recorded, two glow 1amps,. means for Operating one lamp on the positive half of the wave, meansfor preventing the operation of this lamp on the negative half of the wave, meansfor Operating the other lamp on the negative half, means for preventing the operation of this other lamp on the positive half of the wave, and asource, of potential maintaining a glow in each lamp.

. 9. An electric circuit for push-pun sound recording comprising an input circuit, two threeelectrode. glow lamps means connecting: a pair of& electrodes of one lamp across a portion of the input circuit, means connecting a pair of electrodes of the other lamp across a portion of the input circuit, a source of potential biasing said pairs oppositely just below the extinction point and means including the third electrode maintaining a glow in each lamp.

10. In push-pun sound-on-film recording, two glow lamps operatingrespectively on positive and negative half-wave rectifications of the sound to be recorded and means maintaining a pilot glow ineach lamp` having a `predetermined intensity sufiicient uniformly to expose the film beyond the non-useful portion of the toe of its characteristic curve whereby all additional exposure caused by increased glow due to the sound input will be recorded on a useful portion of said characteristc curve.

JOHN H. MCLEOD.

, RICHARD s; MORSE. 

